Tourists react after kissing a gray whale calf at the San Ignacio Lagoon, Baja California Sur state, Mexico on March 1, 2010. Although a debate is now raging among some whaling nations to begin limited hunting again, the Pacific gray whales have been protected since 1947, and are at the center of a growing whale-sightseeing industry. Their numbers have dropped by a third, from around 26,000, in the late 1990s. Scientists say that the decline was caused by melting artic ice impacting on their food chains, which include small fish, crustaceans, squid and other tiny organisms. A small-scale whale-sightseeing industry was developed in the remote spot of San Ignacio Lagoon, off Mexico's northwest Baja California peninsula, where grey whales breed and nurse their calves each year after migrating thousands of miles from Canada and Alaska. AFP PHOTO/OMAR TORRES (Photo credit should read OMAR TORRES/AFP/Getty Images)

Photo: Omar Torres, AFP/Getty Images

Tourists react after kissing a gray whale calf at the San Ignacio...

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Two gray whales show their rostrums at the San Ignacio Lagoon, Baja California Sur state, Mexico on February 28, 2010. Although a debate is now raging among some whaling nations to begin limited hunting again, the Pacific gray whales have been protected since 1947, and are at the center of a growing whale-sightseeing industry. Their numbers have dropped by a third, from around 26,000, in the late 1990s. Scientists say that the decline was caused by melting artic ice impacting on their food chains, which include small fish, crustaceans, squid and other tiny organisms. A small-scale whale-sightseeing industry was developed in the remote spot of San Ignacio Lagoon, off Mexico's northwest Baja California peninsula, where grey whales breed and nurse their calves each year after migrating thousands of miles from Canada and Alaska. AFP PHOTO/OMAR TORRES (Photo credit should read OMAR TORRES/AFP/Getty Images)

Photo: Omar Torres, AFP/Getty Images

Two gray whales show their rostrums at the San Ignacio Lagoon, Baja...

Image 3 of 4

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A gray whale's flukes are seen at the San Ignacio Lagoon, Baja California Sur state, Mexico on February 28, 2010. Although a debate is now raging among some whaling nations to begin limited hunting again, the Pacific gray whales have been protected since 1947, and are at the center of a growing whale-sightseeing industry. Their numbers have dropped by a third, from around 26,000, in the late 1990s. Scientists say that the decline was caused by melting artic ice impacting on their food chains, which include small fish, crustaceans, squid and other tiny organisms. A small-scale whale-sightseeing industry was developed in the remote spot of San Ignacio Lagoon, off Mexico's northwest Baja California peninsula, where grey whales breed and nurse their calves each year after migrating thousands of miles from Canada and Alaska. AFP PHOTO/OMAR TORRES ---- MORE PICTURES IN IMAGE FORUM (Photo credit should read OMAR TORRES/AFP/Getty Images)

The season of migration has come again to the warm blue waters off the coast of Mexico. Mother gray whales are nursing their newborn calves, plumping them up for the 6,000-mile trip to summer feeding grounds in the Arctic.

This migration, one of the world's longest, has gone on for thousands of years. But predicting the future of this watery voyage raises questions about how the changing climate is affecting species living in the Arctic, the part of the world being transformed most dramatically by climate change.

As ocean and atmospheric temperatures rise, the gray whales - and other Arctic dwellers like the walrus, polar bear, ice seal and Arctic fox - are finding their habitat and food supplies shift.

But little is known about that food chain, and even less is known about how it will adjust as the climate changes.

The teeming Arctic waters are among the richest in the world but are also the least studied. Months of dark days and impassable frozen seas are two of the difficulties. Researchers want to fill crucial data gaps so that they can advise on how best to safeguard the wild Arctic. Protection is crucial, they say, as the Northwest Passage begins to open year round.

The pack ice is melting earlier and forming later, scientists say. Plankton, crustaceans and fish, all food for other wildlife, reproduce at the dynamic edge of the sea ice, where it floats over shallow waters near shore. When melting causes that edge to move off the continental shelf into deep open ocean waters farther north, the marine organisms that feed larger wildlife become out of reach, scientists say.

That's a problem for the gray whales heading north from the haven of Baja California's waters.

The whale's favorite fatty marine crustacean, the amphipod, has declined in the whales' Bering Sea feeding grounds over the past 30 years. Whales with their babies are forced to swim through the Bering Strait and fan out farther into the Arctic Ocean searching for a substitute food supply.

In recent years, scientists at the National Oceanic and Atmospheric Administration say, gray whales, which typically sift out little crustaceans from the bottom are now eating shrimp-like crustaceans and even krill in ocean waters. Scientists say they have to eat tons more of them to make up the lost mass of fatty amphipods. "It's like replacing steak with vegetables," said one researcher.

Since 1984, most gray whale estimates have been between 20,000 and 24,000, with a dip to 17,000 in 1992-93 and a high of 26,000 in 1997-98. Scientists link the swift transformation of the Bering Sea ecosystem to the decline and don't yet know if eating a wider diet will be enough to stabilize the population.

"It's very important to see what's happening up there in the Arctic," said biologist Steven L. Swartz, who co-directs the Ecosystem Science Program at San Ignacio Lagoon in Baja California. "The gray whale's old prey fields are gone. I want to know where they can find food."

"This is a fragile ecosystem that needs to be protected but is also going to be used," said Doug DeMaster, research science director for NOAA Fisheries in Juneau, Alaska.

Environmental groups and scientists have been pressing for more research before industrial development, including plans for new oil drilling on Alaska's North Slope, forges ahead.

"I do know that the (1989) Exxon Valdez spill was very difficult to clean up in an area that was ice-free. We don't yet know how to clean up oil spills in broken pack ice," said Melanie Smith, a biologist with the National Audubon Society in Anchorage. "We need to understand where the key habitats are located, the places that are essential to ecosystems. If they were severely impacted, there would be a ripple effect."

Back in San Ignacio Lagoon, only the mothers and babies are making ripples now. Most of the single females and males have left for the long journey north. The mothers and babies remain, packing pounds on the young, preparing for the uncertainty ahead.

About gray whales

Gray whales (Eschrichtius robustus) are protected by international law after being hunted to the edge of extinction last century. They are often covered with parasites and other organisms that make their snouts and backs resemble crusty rocks. Some basic facts:

Population: Between 20,000 and 24,000.

Size: 40 to 50 feet.

Weight: 30 to 40 tons.

Diet: Mostly tiny, shrimplike crustaceans and tube worms. They are the only bottom-feeding whale.